pku 1753

#include<iostream>
#include<cstdio>
#include<queue>
using namespace std;
const int maxn = 65535;
int vis[maxn+10];

int f(int sum, int i){
    sum ^= 1 << (15 - i);
    if (i - 4 >= 0)                             sum ^= 1 << (15 - i + 4); 
    if (i + 4 <= 15)                            sum ^= 1 << (15 - i - 4); 
    if (i != 0 && i != 4 && i != 8 && i != 12)  sum ^= 1 << (15 - i + 1);
    if (i != 3 && i != 7 && i != 11 && i != 15) sum ^= 1 << (15 - i - 1);
    return sum;
}

void bfs(int sum){
    memset(vis, 0, sizeof(vis));
    queue<int>ss;
    ss.push(sum);
    while (!ss.empty()){
        int x = ss.front();        ss.pop();
        if (x == 0 || x == maxn){
            printf("%d\n", vis[x]);
            return;
        }
        for (int i = 0; i < 16; ++i){
            int k = f(x, i);
            if (!vis[k]){
                vis[k] = vis[x] + 1;
                ss.push(k);
            }
        }
    }
    printf("Impossible\n");
}


int main(){
    char st[10];
    int cnt = 0;
    for (int i = 0; i < 4; ++i)
    {
        scanf("%s", &st);
        for (int j = 0; j < 4; ++j)
            cnt = (cnt << 1) + (st[j] == ‘b‘ ? 0 : 1);        //状态压缩
    }

    bfs(cnt);
    return 0;

}